Puncoviscana 600 - 540 Ma
GD 531-586 Ma
Fig. 12.Tectonic setting of Laurentia, Baltica and Amazonia at the end of the Neoproterozoic, according toTegner et al. (2019), showing the Central Iapetus Magmatic Province, and associated mafic and kimberlite assemblages (white dashed line; modified fromTegner et al., 2019). ED: Egersund Dyke Complex (Bingen et al., 1998), GD: Grenville Dyke Complex, LR: Long Range Dyke Complex, SD; Scandinavian Dyke Complex (Tegner et al., 2019).
30°S IAPETUS 535 Ma Early Cambrian
IAPETUS
IAPETUS 500 Ma Late Cambrian
30°S
RHEIC 450 Ma Middle Ordovician
RHEIC
415 Ma Late Silurian 370 Ma Late Devonian
30°S 30°S
30°S
340 Ma Early Carboniferous
PALAEOTETHYS 30°S
30°S 280 Ma Early Permian
El Tigre Fm. deposited along Iapetus passive margin El Tigre Fm. deposited along Iapetus passive margin
ET ET
Arco Iris Unit, deposited in Famatinian back-arc?
Merida Santander Quetame La Miel
Cuche Palenque Pumbuiza Establishment of a passive margin
Cuche Palenque Pumbuiza Cabanillas
Guaquia Chiguinda La Victoria Isimanchi Ambo
Rovira Malacatos
Sedimentation within a Carboniferous back-arc? Active margin record (now segmented) in N. Andes
A B
C D E
F G
Fig. 13.Plate reconstructions for the Palaeozoic margin of northwestern Gondwana, modified and simplified fromCocks and Torsvik (2006)andVan der Lelij et al. (2016). ET:
El Tigre unit.
this is consistent with i) Phanerozoic rift-to-drift transitions, which typ-ically span ~40 Ma, and ii) the conclusions ofTegner et al. (2019), who compiled geochemical and geochronological data from the CIMP to sug-gest that it records a rift-to-drift transition during ~616–590 Ma. Fur-thermore, despite the paucity in geochemical data, a comparison of the elemental and Nd isotopic compositions of the Scandinavian and Egersund Dyke complexes with the Huarguallá Gabbro unit sug-gests these ultramafic and mafic rocks formed with a mantle plume setting at ~616 Ma, which progressed towards early continental rifting within ~50 Ma.
Famatinian arc magmatism (Figs. 13a – d) commenced along western Gondwana at ~530 Ma. We consider the lack of Cambrian and Ordovician magmatism in Ecuador to be a consequence of its inboard, back-arc position (Fig. 13c) due to the presence of basement rocks that now form part of southwestern North America (e.g.Spikings et al., 2016). A detailed reconstruction is beyond the scope of this work, although Ordovician magmatism is recorded in the Maya Block (e.g.Solari et al., 2010) and the Acatlán Complex (Keppie et al., 2008), which rifted from Gondwana during 245–216 Ma (Spikings et al., 2016). Abundant Ordovician magmatism within the Merida Andes and Santander Massif, along with minor exposures in the Central Cordil-lera, Quetame and Floresta Massifs of Colombia suggests that these rocks originated within an arc zone located north of the outboard ter-ranes (Fig. 13c), which was distal to the Triassic rift. This interpretation is consistent with the lack of Triassic rift-related rocks in these units. The oldest Ordovician magmatism recorded in the outboard blocks occurred at ~480 Ma, suggesting these blocks may have faced a Cambrian passive margin (e.g.Fig. 13b), and subduction was delayed relative to regions to the west (e.g. Eastern Cordillera of Peru), and south (e.g. Merida Andes of Venezuela). Thus, the weakly metamorphosed quartzites, wackes and lutites of the Cambrian El Tigre Fm. (maximum depositional age 512 ± 21 Ma;Suhr et al., 2019) may have been deposited in a passive margin, which received detritus from distal arcs.
Sandstones of the Devonian(?) Pumbuiza Unit that forms part of the basement of the Oriente Basin, and the Palenque Unit (maximum depo-sitional age of 391 ± 17 Ma;Suhr et al., 2019), which is exposed in the northern Amotape Massif may be coeval with deposition of the Cabanillas Gp. in Peru, and the Floresta, Cuche and Tibet Fms. in Colombia, while no Devonian units have been identified in the Merida Andes (Fig. 10). This period coincides with a Devonian magmatic gap and a passive margin environment along western Gondwana (Fig. 13e), which is reflected in a paucity of Devonian-aged detrital zir-cons in Carboniferous basins in Ecuador (this study), Peru (Chew et al., 2007;George et al., 2019) and farther south (e.g.Bahlburg et al., 2009).
However,Horton et al. (2010)found a peak in Devonian-aged detrital zircons in Devonian sedimentary rocks in Colombia, with high Th/U ra-tios that allude to an igneous provenance, leading them to conclude that a Devonian arc may have shed detritus into northwestern Gondwana.
With the exception of a rhyolite within the volcanic Bladen Fm.
(406 + 7/−6 Ma; Martens, 2010), and granites of the Mountain Pine Ridge (420–405 Ma;Steiner and Walker, 1996) of the Maya Mountains, Maya Block, Devonian-aged magmatism within southwestern North America is sparse. However,Martens et al. (2010)suggest that Devo-nian magmatism may be extensive in the subsurface of the Maya Block, although a lack of geochemical data precludes a tectonic interpretation.
Carboniferous magmatism occurred within the Acatlán and Oaxacan complexes, and the Maya Block during 334–289 Ma (Kirsch et al., 2012;
Ortega-Obregón et al., 2014;Solari et al., 2010), and the Eastern Cordil-lera of Peru during 360–285 Ma (Miskovic et al., 2009), although it is lacking in the present-day cordilleras of Ecuador and Colombia (Fig. 10), presumably due to an inboard, back-arc position (Fig. 13f).
The lack of Carboniferous magmatism in the Merida Andes (Van der Lelij et al., 2016) reflects the consumption of Rheic oceanic lithosphere beneath the North American Plate (e.g.Nance et al., 2010), and passive margin conditions along northern South America. Siliclastic rocks were
deposited within Carboniferous basins in northwestern Gondwana, which have been subsequently tectonically dissected, forming the siliclastic Chiguinda and Isimanchi (Cordillera Real), and La Victoria (Amotape Massif) Fms. within Ecuador, and the El Imam and Guatiquia Fms. in Colombia (Fig. 10).
Carboniferous to Permian sedimentary successions in the Merida Andes (Fig. 10) have been interpreted to form part of a foreland basin (Laya and Tucker, 2012) that formed during the closure of the Rheic Ocean and the development of the Alleghenian Orogen (Fig. 13g). How-ever, the tectonic setting of Carboniferous units in Ecuador and Colombia has not been well constrained, while Permian sedimentary rocks have not been identified within the cordilleras.Vinasco et al.
(2006)andPiraquive (2017)suggested that a regional metamorphic event affected Permian intrusions in the Cordillera Central and Sierra Nevada de Santa Marta, respectively, at ~278 Ma. However, there is no clear evidence that zircon growth at ~278 Ma had a metamorphic origin (e.g. Th/U ratios are close to 1; Piraquive et al., 2017). On the contrary, garnets within the La Secreta Mylonites of the Inner Santa Marta Meta-morphic Belt (northern Colombia) reveal clear evidence for regional scale metamorphism at ~250 Ma, while contemporary zircons with Th/U ratios of ~0.1 supports a metamorphic origin (Piraquive, 2017), which coincides with high-temperature metamorphism in the Maya (Weber et al., 2007) and Chortis (Ratschbacher et al., 2009) blocks. Col-lisional events at ~250 Ma (Spikings et al., 2016) may be responsible for Permian sedimentary gaps in the Cordilleras Real (Ecuador) and Central (Colombia), as a distal response to continent-continent collision. Perm-ian arc magmatism occurred during 298–253 Ma within northwestern Gondwana (including the contemporary conjugate margin that cur-rently forms part of the North America Plate), although the abundance of Permian intrusions decreases from northern Colombia to Ecuador.
Only one Permian magmatic rock has been identified within Ecuador (Rovira Complex;Spikings et al., 2016;Fig. 10), and thus it is likely that present-day Ecuadorian continental crust mainly existed in the Permian back-arc.
6. Conclusions
1. The Huarguallá Gabbro unit is defined as a series of olivine bearing gabbros and ultramafic rocks exposed in fault bounded slivers of the anastomosing Peltetec Fault Zone along the westernflank of the Cordillera Real. Argon isotope data from plagioclase yields
40Ar/39Ar dates that span between 565.5 ± 34.4 Ma and 581.8 ± 41.1 Ma, which are interpreted as crystallisation ages and reveal the presence of late Neoproterozoic mafic rocks, regardless of the poor precision. Geochemical compositions are consistent with an as-thenospheric magma source that was contaminated with ~25% of continental crust in a rift setting. The Huarguallá Gabbro forms part of the geographically dispersed Central Iapetus Magmatic Province, and its location in Ecuador is consistent with the continental recon-struction ofTegner et al. (2019), which juxtaposes northwestern Gondwana with late Neoproterozoic dyke complexes within Baltica.
Stratigraphic constraints on the formation of rift-basalts of the Puncoviscana Belt (Argentina), suggests they formed within the same setting, during the rift stage of the Iapetus Wilson Cycle.
2. The Cambrian, siliclastic El Tigre Unit of the Amotape Complex in Ecuador may have been deposited prior to the onset of arc magmatism in either the conjugate margins of the Maya Block and Mixteca Terrane, and to the west (Peru) and south (Venezuela), and thus is considered to be a passive margin sequence.
3. Ecuadorian lithosphere was in the back-arc during the Ordovician, while the arc rocks are currently preserved in dispersed locations within the conjugate margins of the Maya Block and the Acatlán Complex (Mixteca Terrane). Consequently, we do not predict that Ordovician arc rocks form part of the contemporary basement to Ecuador. Ordovician intrusions form a substantial component of the basement of the Merida Andes, Santander Massif, and dispersed
inliers in the cordilleras of Colombia. These crustal blocks may have been located north of the outboard conjugate Maya, Mixteca and Oaxaquia blocks, and thus resided within the Ordovician arc, which was dismembered by younger tectonic events.
4. Maximum depositional age constraints combined with fossil assem-blages suggest the Chiguinda and La Victoria siliciclastic units were deposited during the Carboniferous. The complete lack of Carbonifer-ous plutons in Ecuador and Colombia is attributed to a back-arc posi-tion to the relict conjugate margins of the Maya Block, Oaxaquia and the Mixteca Terrane, while some Carboniferous detritus may have been sourced from the Carboniferous arc in Peru. The lack of Carbon-iferous arc magmatism in the Merida Andes reflects the subduction of remnants of the Rheic Ocean beneath Laurentia during the amal-gamation of Pangaea.
Supplementary data to this article can be found online athttps://doi.
org/10.1016/j.gr.2020.10.009.
Credit author statement
Richard Spikings: Raised funds, devised the concept, student super-vision, sampled rocks, data acquisition and reduction, wrote the manuscript.
Andre Paul: Sampled rocks, data acquisition and reduction.
Cristian Vallejo: Field work collaborator.
Pedro Reyes: Sampled rocks, Field work collaborator.
Declaration of Competing Interest
The authors declare that they have no known competingfinancial interests or personal relationships that could have appeared to infl u-ence the work reported in this paper.
Acknowledgements
The authors are grateful for the assistance of Bernado Beate during field work and logistical planning in Ecuador. The manuscript was im-proved by the useful and thorough reviews of Dr. Sarah George and an anonymous reviewer. Funds for the project were awarded to RS by the Swiss National Science Foundation (grants 200020_134443 and 200020_146332).
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